This invention relates to the spiral wrapping of a strip of material around an axis and, more specifically, to the winding of a prepunched strip of material around a rotating mandrel in order to produce a motor core with radial slots.
A spirally wound core with radial slots is a requirement for the stator of an axial airgap motor. The cores are produced by winding a long, thin strip of steel around a mandrel or arbor. As the strip is wrapped around the rotating mandrel, the radial thickness of the toroidally shaped core continually increases until the finished size is achieved.
Since motor stators are designed to allow conductive coils to be wound within them, they are typically equipped with a plurality of radial slots in one axial face of the core. There are two ways to produce a core with radial slots. One way is to wind a slotless core and then machine the radial slots after it is wound. An alternative is to punch the slot shape into the metal strip with a punch press before winding so that the consecutive punched layers of the metal strip form the radial slots by aligning themselves with corresponding slots in preceding, radially inboard layers as the core is wound.
The machining method has two significant negative characteristics inherent in its use. It is very costly since it requires two separate and distinct serial operations to be performed on the core and, secondly, the resulting machined inner surfaces of the radial slots are not ideal for motor core performance. These inner surfaces, which are intended to be made up of many individual thin edges of the strip material, tend to become smooth and homogeneous from the machining operation, significantly worsening the eddy current loss characteristics of the core.
The prepunching alternative is made difficult by the ever increasing diameter of the core as it is being wound on the mandrel. Since each layer of material is wrapped around all preceding layers, its effective diameter is larger than that of previous layers with a resulting larger circumference. Each of the radial slots passes through each layer one time and, therefore, the number of slots through each layer is identical to that of every other layer. Because of the above-mentioned increasing circumference of outer layers and the slot number identity, the distance between each slot in the strip of material must be increased from that of the preceding slots. If these increasing dimensions are not accurately maintained, the resulting slots will be skewed instead of radially straight. Although some motor designs require skewed slots, the slots of any motor, whether with radial or skewed slots must be accurately produced according to the motor's design.
Stacking factor is another characteristic of laminated composites which severely exacerbates the skewing problem and is known to those skilled in the art. The stacking factor of a given material is defined as the arithmetic sum of the thicknesses of a selected number of pieces of the material divided by the measured stack height of that selected number of pieces. The stacking factor varies as a function of material thickness, coating and surface condition and is normally less than one.
Existing methods of motor core manufacture compensate for slot skew by coil winding techniques or by designing the overall motor to allow for the inherent inefficiency of skewed stator core slots. The present invention has as its prime objective the winding of axial airgap stator cores, with prepunched strips, that have straight and radial slots and the capability of creating a predetermined amount of skew when that characteristic is desired.